World denounces North Korea nuclear test

Washington: The US, South Korea and Japan are sniffing the air in a coordinated intelligence effort to determine what type of nuclear device was detonated by North Korea's secretive regime.

The country's two previous underground nuclear tests, in 2006 and 2009, drew on its limited reserves of plutonium. Any evidence that North Korea used highly enriched uranium this time would signal that it has developed a second source of fissile material, expanding its potential warhead capabilities and raising the risk that the cash-strapped nation may sell such uranium to would-be nuclear weapons states such as Iran.

Air-sampling equipment, some in aircraft and some at ground-based facilities, are sniffing for residual radioactive emissions from the underground tests that would confirm it was a nuclear explosion and provide markers for the type of fissile material used. A similar inquiry was successful in 2006, while one in 2009 failed because that test didn't vent radioactive material.

People watch a television news report on U.S. President Barack Obama's State of the Union address at Seoul Railway Station in Seoul, South Korea. Photo: AP

"One important question is whether the nuclear test used only plutonium or involved highly enriched uranium, either alone or in combination with plutonium," said David Albright and Andrea Stricker of the Institute for Science and International Security said.

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North Korea tested "a smaller and light" nuclear device yesterday, the official Korean Central News Agency said, two months after test-firing a long-range rocket. The device had a yield of 6 to 7 kilotons, South Korea's Defense Ministry estimated. That was bigger than the previous two North Korean detonations, though less than half the explosive power of the uranium-fuelled bomb that the US dropped on Hiroshima, Japan, in 1945.

Size, Type

Much of the policy debate since the blast yesterday has focused on indications that North Korea was testing a miniaturised nuclear device as the regime seeks to develop a warhead that could be placed on a long-range missile. The US intelligence community assesses that North Korea remains some years from achieving the capability to threaten the continental US, though it may pose a nuclear threat to its regional neighbours sooner.

North Korea said in its statement that the nation now has achieved a "diversified" nuclear capability -- a comment that drew attention as a possible signal that highly enriched uranium was used.

That would be a significant development, according to James Acton, a nuclear policy specialist at the Carnegie Endowment for International Peace, a Washington-based policy group.

Small Stockpile

"North Korea's plutonium stockpile is small and has been depleted somewhat by earlier tests," Mr Acton said. "Given the dilapidated state of its plutonium-production infrastructure, it would be both difficult and expensive to produce more. If, however, North Korea has mastered uranium enrichment, it could expand its arsenal relatively cheaply and quickly."

North Korea's plutonium supply is limited because it shut down and disabled the sole source, a reactor complex at Yongbyong, under a short-lived 2007 denuclearisation accord with the U.S., South Korea, Japan, China and Russia.

North Korea was estimated last year to have 30 to 50 kilograms of separated plutonium produced before the shutdown, enough for at least half a dozen nuclear weapons, according to a US Congressional Research Service report.

"While North Korea's weapons program has been plutonium- based from the start, in the past decade, intelligence emerged pointing to a second route to a bomb using highly enriched uranium," according to the report last year.

After repeatedly denying such an effort, North Korea in November 2010 showed visiting American experts early construction of a 100-megawatt light-water reactor and a newly built gas centrifuge uranium-enrichment plant, both at its Yongbyong site. In late 2011, North Korea said the facility was producing uranium enriched to as much as 3.5 percent, suitable for use in a power reactor but short of the 90 percent enrichment for a nuclear device.

"The North's disclosure supports the United States' longstanding assessment that North Korea has pursued a uranium- enrichment capability," U.S. Director of National Intelligence James Clapper told Congress a year ago.

The current detection effort involves the US and its allies, as well as global monitoring stations established under the Comprehensive Nuclear Test-Ban Treaty, which prohibits nuclear testing in an effort to slow the spread of nuclear weapons.

Kerry Comments

"If radioactivity is detected, it should be possible to distinguish between a uranium and a plutonium device," Mr Acton said. "It may also be possible to assess what other materials were present in the device and hence make educated guesses about its design."

Secretary of State John Kerry said Wednesday that North Korea's nuclear test poses a threat to the US and to global peace.

"The international community needs to come together for a swift, clear response," Mr Kerry said in Washington. Mr Kerry said the response to North Korea would have to be swift and clear because it also would send a message to Iran.

"This is not only about the DPRK and its continued flaunting of its obligations under three separate Security Council resolutions," Mr Kerry said. "This is about proliferation, and it's also about Iran because they're linked."

The US has said that Iran is using its uranium-enrichment technology as part of an effort to develop nuclear weapons capability, and the US and Israel have looked for evidence that Iran is cooperating with North Korea in weapons development.

Iran denied such cooperation in December, following a report by Japan's Kyodo news agency saying Iran had stationed defense staff in North Korea for joint work on missile and nuclear development.